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Scientists Find Cells In The Human Brain Responsible For Episodic Memory

AUDIE CORNISH, HOST:

A lot of memories are like short movies. If you fall off a bike, your brain will probably record the entire sequence of events that put you in pain. That's known as an episodic memory. And now scientists say they have identified cells in the human brain that make this sort of memory possible. NPR's Jon Hamilton has more.

JON HAMILTON, BYLINE: They're known as time cells, and they were discovered in rodents years ago. But a team of researchers wanted to see if these cells also exist in humans. So they studied the brains of 27 people attempting a difficult memory task.

BRAD LEGA: This type of memory task is not one that, like, a rodent would be able to do.

HAMILTON: That's Dr. Brad Lega, a neurosurgeon at the University of Texas Southwestern Medical Center in Dallas. Lega says participants were asked to study sequences of words on a laptop computer.

LEGA: The words appear on the screen one after the other, about 12 to 15 items at a clip. They're separated by a couple of seconds.

HAMILTON: Then after a break, people were asked to remember the words. Meanwhile, scientists were measuring the activity of individual cells in the hippocampus and another brain area involved in the perception of time. This was possible because the people in the study already had electrodes in their brains as part of a treatment for severe epilepsy. Lega says the team discovered certain cells that would fire at specific times during each sequence of words.

LEGA: The time cells that we found - they're marking out discrete segments of time within this, like, approximately 30-second window.

HAMILTON: Time stamps that help people recall when they had seen each word and in what order. Lega says the finding suggests that the brain uses the same approach when we're reliving an experience, like falling off a bike. We remember the wind in our hair, then seeing the pebble on the road, then the pain.

LEGA: So by having time cells create this indexing across time, you can put everything together in a way that makes sense.

HAMILTON: The time cell study appears in the journal Proceedings of the National Academy of Sciences. Gyorgy Buzsaki of New York University says it's important, even though the result was predicted by experiments in animals.

GYORGY BUZSAKI: The final arbitrator is always the human brain.

HAMILTON: Buzsaki says the study helps explain the memory limitations found in people who have damage to the hippocampus. In one experiment, he says, scientists compared the memories of a group of people who had just finished a tour of a university. Buzsaki says the people without hippocampal damage all told pretty much the same story.

BUZSAKI: First thing is we have seen the fountain, and then there was a little girl who fell off the bicycle and so on. And these sequences are completely and absolutely gone in people with hippocampal lesions.

HAMILTON: Probably because their brains don't have time cells to recreate a sequence of events. But Buzsaki says time cells aren't like clocks. Their pace is constantly changing depending on factors like mood.

BUZSAKI: When you have to wait for the elections, then, you know, every day is a long day. The same thing is when we're asking, you know, when is COVID over? It's very, very slow. But when you're having a good time, time flies.

HAMILTON: Buzsaki says, as a result, our perception of time isn't very reliable.

Jon Hamilton, NPR News.

(SOUNDBITE OF JAYDA G'S "SOUND OF FUCA") Transcript provided by NPR, Copyright NPR.